Technical Field
The present invention relates to a carrier, a two-component developer, a developer for replenishment, a process cartridge, an image forming apparatus, and an image forming method.
Description of the Related Art
Electrophotographic image forming methods include forming an electrostatic latent image on an image bearer such as a photoconductive material, transferring a charged toner thereto to form a toner image, transferring the toner image onto a recording medium such as paper, and fixing the toner image thereon to form a final output image. Recently, electrophotographic copiers and printers are rapidly developing from monochrome to full-color, and full-color markets are expanding.
The electrophotographic image forming methods typically include overlaying three primary color toners, i.e., yellow, magenta, and cyan toners, or four color toners including the above-described three toners and black toner, to reproduce all colors. Therefore, to produce a sharp full-color image having good color reproducibility, the surface of a fixed toner image has to be smooth to reduce light scattering. This is why many conventional full-color copiers produce images having medium to high glossiness of from 10% to 50%.
As a method of fixing a dry toner image on a recording medium, contact heat fixing methods using a heated roller or a heated belt having a smooth surface are typically used. Although the methods have high heat efficiency, and are capable of fixing at high speed while imparting gloss and transparency to color toners, offset problems do occur, wherein a part of a toner image adheres to a fixing member and then is transferred to another image, during separation of the fixing member from the melted toner image after pressing the surface of the fixing member.
For the purpose of preventing offset problems, the surface of the fixing member has typically been coated with silicone rubber or a fluorine-containing resin. Further, a releasing agent in the form of an oil such as silicone oil is applied to the surface of the fixing member. Although this approach is quite effectively used to prevent offset problems, a release oil applicator is required and the resultant fixer becomes larger.
Therefore, as an alternative, there are oilless arrangements or systems that apply only a small amount of oil. Such system use a monochrome toner having high viscoelasticity when melted so as not to break down internally, and including a release agent such as wax.
In addition, even full-color image forming apparatuses are becoming oilless as well for the purpose of downsize and simplification of the fixer. However, as mentioned above, to improve color reproducibility of a color toner, the color toner needs to have lower viscoelasticity because the fixed color toner image is required to have a smooth surface. Therefore, the color toner has offset problems more often than the monochrome toner does, making it more difficult to make a fixer oilless or use only a small amount of oil. In addition, a toner including a release agent has higher adherence to an image bearer and lower transferability to a transfer paper. Further, the release agent therein contaminates friction-charged members such as a carrier and lowers the chargeability thereof, resulting in deterioration of durability of the toner.
On the other hand, for the purpose of preventing toner constituents from filming, making the surface thereof uniform, preventing oxidization thereof, preventing deterioration of moisture sensitivity thereof, extending lives of developers, preventing adherence of the carriers to the surfaces of photoreceptors, protecting photoreceptors from being damaged or abraded by the carriers, controlling charge polarity thereof and controlling charge quantity thereof, a resin having a low surface energy such as fluorine-containing resins and silicone resins is applied on the carrier core material to prolong the life of the carrier.
However, recently, image forming apparatuses have been desired to further downsize, to reduce environmental load that tends to increase due to longer life, to stably produce quality images for long periods, and to reduce cost of producing one image. Therefore, a carrier having higher durability is required. This is because a compact image forming apparatus includes an image developer containing a developer less and a carrier thereof receives stress more.
Properties of the carrier need to be kept stable for a long period of times to produce quality images for a long period of time. Particularly, a resistance value which is the main property of the carrier is preferably maintained. Carbon black has conventionally and preferably been used to control the resistance value of the carrier because a small amount of carbon black can control the resistance value at low cost. However, when the carbon black is transferred to the toner, an image which is somewhat black is produced. Therefore, carbon black needs to skillfully be used to produce full-color images.